This invention relates generally to sampling techniques to determine Doppler frequencies, and more specifically to a radar altimeter which incorporates sampling techniques that overcome zero Doppler returns.
Known pulse Doppler radar altimeters process the received radar returns at a base band frequency to minimize the amount of data processed and to minimize processing speeds required to accurately process the radar returns. In such radar altimeters, only a Doppler shifted frequency remains after the radar return signal is converted to a base band frequency. However, under stationary conditions, for example, an aircraft parked on a runway, there is zero Doppler frequency shift. A zero Doppler frequency shift results in a DC level signal being provided to the base band processing circuits. The result is that the base band processing circuits provide no data that can be utilized to determine an altitude.
It is desirable that a radar altimeter operate properly with the aircraft stationary on the ground to allow preflight testing and calibration. Other attempts have been made to rectify the zero Doppler frequency shift problem. One known solution to the zero Doppler frequency shift problem includes mixing an output of a RF mixer local oscillator with a signal source whose signal output is approximately equal to the frequency of a Doppler shift that is typically encountered during a flight. However, this solution requires that additional RF circuitry be added to the radar altimeter, which adds significant expense to the radar altimeter.
In one aspect, a method for simulating a Doppler signal under stationary conditions is provided. The method comprises sampling a radar return signal at an integer multiple of the return signal frequency plus a fraction of the return signal period and generating a base band signal from the samples.
In another aspect, a radar altimeter is provided. The radar altimeter comprises a memory, a sequencer for modulating a first signal, a transmitter coupled to the sequencer for transmitting a radar signal including the modulated first signal toward the ground, and a receiver for receiving a reflected radar signal from the ground, the received radar signal including the modulated first signal The altimeter also comprises a digitizer coupled to the receiver for generating digital samples of the modulated first signal at an intermediate frequency (IF), the IF digital samples being stored in the memory and a processor coupled to the memory which samples the stored IF digital samples at an integer multiple of the period of the stored IF digital samples plus a fraction of the period of the stored IF digital samples.
In still another aspect, a decimation frequency logic unit for a radar altimeter is provided. The unit is configured to provide a sampling rate to an external device receiving an IF signal, the sampling rate at a frequency whose period is at an integer multiple of the period of IF signal plus a selected fraction of the period of the IF signal.